1. Field of the Invention
The invention relates to an exhaust gas turbocharger having a housing and having a shaft which is arranged so as to be capable of rotating about its longitudinal axis in the housing and on which a turbine wheel and a compressor wheel are seated and which is guided in radial bearings which are embodied as magnetic bearings and in at least one axial bearing, the bearings each having a bearing plate which is seated on the shaft and at least one stator which lies axially opposite said bearing plate on at least one side, thus forming a gap between the bearing plate and the stator. The invention also relates to a method for cooling the bearings of such an exhaust gas turbocharger.
2. Description of the Related Art
Exhaust gas turbochargers are used to improve the efficiency, and thus increase the performance, of internal combustion engines. They have a shaft which is provided at one end with a turbine wheel and at the other end with a compressor wheel. The turbine wheel has the exhaust gas stream of the internal combustion engine applied to it, essentially part of the thermal energy of the exhaust gas being converted into a rotary movement by the turbine wheel. The compressor wheel is driven by means of the shaft, sucks in fresh air and allows it to flow under excess pressure into the inlet ducts of the internal combustion engine, thus improving the volumetric efficiency.
Stringent demands are made of the bearings of the shaft of exhaust gas turbochargers. On the one hand, the shaft reaches high rotational speeds of up to 300 000 rpm. On the other hand, the exhaust gas turbocharger, and thus its bearings, are exposed to high temperatures. A further problem is that the exhaust gas stream which strikes the turbine wheel generates strong axial forces which have to be absorbed in an axial bearing. Owing to the high rotational speeds, the rotating parts of the exhaust gas turbocharger must be balanced highly precisely, so that as few oscillations and vibrations as possible are generated. In addition, it is also necessary to ensure that the very wide temperature range in which an exhaust gas turbocharger operates does not lead to stresses in the bearings owing to material expansion.
Previously, plain bearings or roller bearings were exclusively applied as bearings for the shaft. Owing to the above-mentioned stresses, they are subject to considerable wear and they are responsible, along with their lubrication, for up to approximately 80% of failures of exhaust gas turbochargers. In comparison, a magnetic bearing of the shaft provides the advantage that it is possible to dispense with the use of oil as lubricant. As a result, strict exhaust gas standards for vehicles can be complied with and the reliability of turbochargers can be increased. Furthermore, magnetic bearings provide the advantage of reducing or even avoiding the need to balance the rotating components as these bearings keep the rotor on its center-of-gravity axis. In one respect it is necessary to additionally take into account the magnetic nature of such bearings in comparison with conventional oil-lubricated sliding bearings or roller bearings. While a considerable portion of the heat of the shaft is conducted away by the through-flowing lubrication oil in said conventional bearings, this is not the case with the magnetic bearing. As a result, the air located in the bearing housing is heated. However, the air must not exceed the maximum permitted temperature for the magnets which are used. When NdFeB magnets are used this limit is approximately 130° C.